1.Q: Lens? Window?A: The clear material in front of a flashlight's light source and optical system, intended to protect them, is usually called a lens. However, it is more accurate to call it a "window," as a lens could also refer to some sort of optical system where light goes through a piece of clear glass or plastic and is distorted in some way (e.g. collimated or dispersed). Technically, a window is nothing but a plano-plano lens (flat on both sides), but it's easiest and clearest to call it a window and save "lens" for the above-mentioned optical system.

2.Q: What goes into a flashlight besides the essentials?A: Flashlights aren't just a battery, body, switch, and light source (and sometimes electronics).

One of the most important additions is some sort of optical system, such as a lens or reflector. These focus the light from a source into a narrower beam that can more easily light up distant objects. Reflectors can be made out of plastic, aluminum, steel, or other materials. They must be given a mirror-like surface somehow. Plastic reflectors are very cheap, but can melt in flashlights with an incandescent bulb of more than 15W. Some reflectors are given a slightly textured surface in order to smooth out flaws in the beam, reducing unwanted artifacts. These are known as "stippled," "orange peel," "textured," or "stochastic" reflectors. Optics, or lenses, can take several forms. There is the popular TIR, or Total Internal Reflection, optic, which sits over a light source (usually an LED) and collimates most of its output into its intended beam pattern with little sidespill. A more extreme optic is the aspheric lens, or asphere. Such a lens is simply a plano-convex piece of glass or plastic, with the convex side not quite the shape of a sphere. A spherical convex portion creates chromatic aberration, which basically means a fuzzy image. An asphere can produce precise collimation with an intense hotspot, little corona (if any), and no sidespill. Some lights based on 5mm LEDs will use a basic plano-convex lens to provide a rudimentary level of collimation.

Another important component is the window. Plastic windows are more easily scratched up than glass ones and don't let as much light through, but they are cheaper, lighter, and less likely to break when impacted. Windows can be given anti-reflective coatings which reduce the amount of light reflected back into the flashlight, providing a slight increase in output. There are several glass window types available: mineral glass, sapphire, Borofloat, and UCL are the most common. Mineral glass is simply ordinary glass. Sapphire is literally the mineral (not to be confused with "mineral glass") sapphire, which is very hard and tough, but more expensive than glass. Borofloat (Boro) is a type of glass made by Schott. It is more resistant to thermal shock (repeated heating and cooling), and is thus ideal for high-powered incan hotwires. UCL stands for "Ultra Clear Lens," and is indeed clearer than other types of glass windows, with anti-reflective coatings on both sides. It is not as tough as sapphire or Boro windows, but has the best light transmission properties. According to testing by flashlightlens.com (who sells UCL and Boro lenses), polycarbonate transmits 91% of light directed at it, Boro transmits 94%, and UCL transmits 99%.

A light's O-rings are simple rubber rings which are squeezed between two of the flashlight's components in order to prevent water and other environmental hazards from entering the light. Usually, there is an O-ring just above any threads in the head or tail. When the head or tail are screwed on, they press down on the O-rings to form a seal. Some manufacturers use double O-rings, which just means having two O-rings one after another in a particular part of the light. O-rings work best when they are lubed (see links above).

3.Q: What do filters do?A: A filter is a semi-transparent covering over a flashlight's window that changes the beam in some way. There is a wide variety of filters available for various tasks. A red filter, for example, will turn the flashlight's beam red, which is useful in low-powered flashlights for preserving human night-adapted vision. Remember, however, that traditional white power LEDs don't have much of a red component at all, so a red filter on such a light will significantly reduce output. There are also IR and UV filters, as well as diffusers which soften a flashlight's central spot and provide a wider, more close-range beam. Surefire manufactures many kinds of filters and diffusers for use with their lights, and some can be used on flashlights from other manufacturers.

4.Q: How do I choose a switch?A: The "reverse clickie" is very commonly available in inexpensive and general use household and outdoor flashlights. It has a button which must be fully pressed until it clicks and then fully released in order to get light. Once the light is on, a soft press will momentarily turn the light off, and once the button is released, the light will reactivate. This is a very inexpensive (and hence popular) switch type, but it is not suited for signaling or momentary operation.

A "forward clickie," however, can be gently pressed from the Off position to momentarily activate the light, and it will turn off once the button is released. The user can also push the button until it clicks to keep the light on. This is available in some Maglite, Streamlight, Inova, and Surefire models.

Another type of switch is the Lock-Out Tailcap, or LOTC. Such a switch has no clicking mechanism. It can be "locked out" (prevented from turning on, useful for storage and transport) by unscrewing the tailcap to some degree. If the tailcap is tightened enough, pressing down on it turns the light on until it is released. If it is tightened further, the light turns on and stays on. This type of switch is most common in tactical Surefire models. Some lights have a switch similar to this that cannot be locked out, such as Streamlight's TL-2 and TL-3. They are still suited to tactical use.

The simplest switch is the twisty. In such a flashlight, two components (usually the body and head) are tightened together to close a circuit and turn the light on. These are highly valued for their perceived reliability, but thread quality and frequency of use are a large factor in the system's longevity.

There are several other types of specialty switches, including magnetic reed switches, Hall effect switches, membrane-covered electronic contact switches, and more, but they are best discussed in conjunction with the lights that use them.